Auralex Acoustics

Auralex Acoustics

What is an NRC?

NRC stands for Noise Reduction Coefficient. The method by which NRC is ultimately obtained can be: the Reverberation Room Method (ASTM C423) or the Impedance Tube Method (ASTM C384). The Reverberation Room Method is the more popular of the two in terms of tests conducted on acoustical room treatments. How this method works: Approximately 72 square feet (or more, but not less) of material is placed on the floor of a Reverberation Chamber – a big room with (usually) all hard, concrete surfaces (the opposite of an ”Anechoic” Chamber – a room with no echoes) – and the change in absorption from the empty room to the room with the treatment area on the floor is measured. A kind of ”Before and After” test.

The final result of the calculations is reported as Sabin absorption coefficients (”Sabin alphas” or ”aSAB”) in octave bands from 125 Hz to 4000 Hz. For a convenient, single-number rating, the Sabin alphas for 250, 500, 1000 and 2000 Hz are averaged and the result is the Noise Reduction Coefficient, or NRC. The alphas in the individual octave bands are interpreted as the relative sound absorbed over the octave band range. The higher the number, the more sound is absorbed.

Those of you who are math savvy will notice right away that an NRC doesn’t tell you much. Take a look at the following two columns of alphas with the same NRC:

250 Hz: 0.06 0.36
500 Hz: 0.12 0.36
1000 Hz: 0.48 0.36
2000 Hz: 0.72 0.36
NRC*: 0.35 0.35

*In accordance with standards, NRCs are rounded off to the nearest 0.05.

These two materials have identical NRCs, but do not perform identically in individual bands. If you want to get an idea of how an acoustical material actually performs, look at the alphas in the individual bands. If you want to get ”in the ballpark,” then you may find the NRC useful. NRCs are handy when comparing materials side-by-side, but only to a point. For example, drywall has an NRC of about 0.20 and 2” Studiofoam NRC is 0.80. Obviously, the Studiofoam is better at absorbing sound. On the other hand, 2” Studiofoam and 1”, 6# fiberglass both have NRCs of 0.80, but the alphas in different bands are not the same.

Some other useful absorption coefficient and NRC information:

• The ASTM C423 standard makes it possible to arrive at absorption coefficients and, therefore, NRCs that are greater than 1.00. This may be counter-intuitive since many references define the Sabin alpha and NRC as the ”percentage of sound absorbed” by a material. This treatment of alphas and NRCs as percentages, however, is not really accurate. The formulae used in the standard to measure absorption are dependent on: the decay of sound in the test room, room volume, room temperature and the area of Chamber floor covered by the test material. ”Sabin absorption” and ”Sabine alphas” come from the fact that the absorption is calculated using the Sabine equation. Nowhere in the ASTM C423 standard is there a reference to Sabine alphas being equal to percentages of any kind. Therefore, numbers greater than 1.00 are possible. This means that Sabine alphas are simply a representation of the relative amount of sound absorbed by the material. (Relative to the absorption without treatment in the room.) Higher numbers mean more absorption in that frequency band.

[For more on this, please see The Sabins at Riverbank by John Kopec. Also, the absorption coefficient, or alpha, of a material is sometimes calculated using the difference between incident and reflected sound intensity divided by the incident sound intensity. In other words, a percentage. Care should be taken not to equate this alpha with the Sabin alpha described above (We have made this mistake ourselves in the past!), as they illustrate two different properties. This may be the source of some of the confusion about absorption coefficients.]

f=c/4t

where:

f = low frequency cutoff (Hz)

c = speed of sound in air (usually about 1130 ft/sec)

t = thickness of absorber (ft)

This is misleading. This would be valid if the panel was only placed at normal incidence to a sound source. ”Normal incidence” means at 0°. A loudspeaker facing parallel to a wall would be considered normal incidence. When was the last time you saw this in a control room? Usually, walls, and hence absorbers, are placed at different angles to the sound source. The angular incidence of the sound on the panel increases the absorbers effective depth. Therefore, lower cutoff frequencies are achievable. To use the earlier example, this is the direct cause of absorption values for 4” materials below 850 Hz – the supposed low frequency cutoff for normal incidence. The bottom line: Absorbers such as 4” Studiofoam, Sunbursts, MAX-Wall, LENRDs, Venus Bass Traps, etc. can all absorb low frequency energy. Feel free to contact us to find out which one is right for you!

To purchase ASTM standards (or simply browse abstracts of the standards), please visit www.astm.org